You can delete any comment that you create on anyone else's blog, as long as you signed in to your Google Account when you left the comment. You can also delete any comments (registered or anonymous) that are left on your own blog, or on another blog for which you have admin privileges.
To delete a comment, first make sure that you're signed in to the correct Google Account (administrator of the blog or author of the comment). Then go to the blog and find the page where the comment is listed. Next to the comment, you should see a link to delete the comment.
Once you click "Delete," you'll be taken to a page where you can review the comment and confirm deletion, or change your mind and cancel. There's also a checkbox that says "Remove Forever" -- if you check that before you delete the comment, no trace of the comment will remain on your blog. Otherwise, the commenter's name and a message saying "this comment has been removed" will show up where the comment used to be. Once a comment has been deleted, there's no way to recover it.
It's not possible to edit comments. If you want to edit your own comment, you can simply delete it and then re-enter it with the changes. (However, the timestamp will be modified to reflect the time of the new comment.)